1. Field of the Invention
The present invention relates to a measuring apparatus for measuring the absolute distance between a reference surface and a test surface.
2. Description of the Related Art
As an apparatus for measuring the absolute distance between a reference surface and a test surface, a wavelength scanning type light wave interference measuring apparatus is known. The accuracy of absolute distance measurement by wavelength scanning is generally low. For this reason, the measurement accuracy is improved by combining relative distance measurement using a fixed wavelength. Hence, the main accuracy factors of the wavelength scanning type light wave interference measuring apparatus are the accuracy of wavelength scanning (wavelength scanning amount), the accuracy of the fixed wavelength, and the measurement accuracy of a phase when measuring the relative distance.
FM heterodyne, an absolute distance measurement scheme using wavelength scanning, measures the intensity of a single interference signal, and calculates the absolute distance from a change in the intensity of the interference signal caused during wavelength scanning. For example, Japanese Patent No. 2725434 discloses a technique of guaranteeing the scanning amount of an atmospheric wavelength using a reference interferometer (that is, based on the length of the reference interferometer) and also guaranteeing a fixed wavelength using a wavelength reference such as an etalon or gas cell in FM heterodyne.
As a technique of measuring the absolute distance more accurately than FM heterodyne, Japanese Patent No. 2810956 discloses a technique of measuring a phase by obtaining a Lissajous' waveform from the intensities of two interference signals with a phase shift of 90°. In Japanese Patent No. 2810956, the scanning amount of an atmospheric wavelength is guaranteed by using a common reference interferometer for the wavelength scanning amount and the fixed wavelength. In addition, the fixed wavelength is controlled to make the atmospheric wavelength constant, thereby guaranteeing the variation in the refractive index of atmosphere.
However, because of a low phase measurement accuracy, the conventional light wave interference measuring apparatus cannot attain a sufficient measurement accuracy (that is, required measurement accuracy) even by combing relative distance measurement. Furthermore, when the phase measurement accuracy is low, the wavelength scanning amount needs to be larger to combine absolute distance measurement and relative distance measurement. This adds constraints on choice of a light source in the interferometer.
The techniques disclosed in Japanese Patent Nos. 2725434 and 2810956 need a reference interferometer. Hence, the arrangement of the light wave interference measuring apparatus becomes complex, and the measurement accuracy is reduced due to the variation in the length of the reference interferometer used as a reference.